Thermostats have been used for many years as a temperature sensitive switch which controls heating and/or cooling equipment for conditioning a space in which the thermostat, or a temperature sensor connected to the thermostat, is placed. In the well known manner, a simple thermostat can be adjusted to establish a temperature set point such that, when the sensed temperature in the conditioned space reaches the set point, the thermostat interacts with the heating and/or/cooling equipment to take suitable action to heat or cool the conditioned space as may be appropriate for the season.
Modern thermostat systems, which take advantage of the ongoing rapid advances in electronic technology and circuit integration, have many features which provide more precise supervision of the heating and/or cooling equipment to achieve more economical and more comfortable management of the temperature of a conditioned space. Many modern thermostat systems include a real time clock, a memory and a data processor to run a process control program stored in the memory to accurately measure the temperature of a temperature sensor disposed in the conditioned space and to send control signals to the heating and/or cooling equipment to closely control the temperature of the conditioned space. Modern thermostat systems can incorporate algorithms in their control program to anticipate and minimize hysterisis or overshoot of the temperature in the conditioned space.
Many modern thermostat systems have a central control device or unit that receives environmental sensor data from one or more local sensors. These sensors are local in the sense that they are connected by short wire or are soldered by printed circuit board connection to other electrical components in the programmable thermostat. These sensors can detect temperature, humidity, or other parameters that may be used in a control program by the central control device to control environmental control equipment. The environmental control equipment (comprising HVAC equipment, among others) responds to signals from the central control device to affect the ambient comfort in rooms of a conditioned space. Typically, a local sensor signal is received by the central control device and its value compared with that of a pre-set setpoint. If the sensor value is sufficiently different from the setpoint, environmental control equipment is activated or de-activated in response thereto.
One well known temperature sensor used as a local temperature sensor is the resistance temperature detector (RTD), or thermistor. The thermistor is used to provide a signal voltage that changes as a function of the change in resistance of the temperature sensor. Negative temperature coefficient thermistors are typically made from a thin coil of semiconducting material such as a sintered metal oxide. Increasing temperature of a semiconductor increases the number of electrons promoted into the conducting band. Positive temperature coefficient thermistors are of the “switching” type, which means that their resistance rises suddenly at a certain critical temperature. The devices can be made of a doped polycrystalline ceramic containing barium titanate and other compounds or from a polymer with carbon grains embedded in it.
The typical thermistor comprises a pair of relatively rigid wires joined at one end to the resistive element. The two other wire ends are free for connection with the thermostat. Those free ends of the thermistor in prior art thermostats are usually soldered to support holes in a printed circuit board. The resulting structure provides a very secure support connection for the thermistor to the circuit board as well as a reliable electrical connection with other electrical components of the thermostat.
A desired design goal in programmable thermostats has been to reduce their size and/or incorporate additional electrical components to add to the functions of the thermostat. For protection of those components and to meet required aesthetic external features, these thermostats have a rigid housing that is adapted to be wall mounted. The housing provides openings for access to user interface features such as push buttons and a liquid crystal display screen. Making thermostats with a small housing is desirable because they are usually prominently wall mounted and might be unsightly if too large. Reduction of overall device size is accomplished in part by increasing the density of components in the device. However, increasing component density results in an increase in the density of energy that must be dissipated from the device as heat.
Consider the following example of heat generation in the housing of a modern thermostat. Relays for actuation of environmental control equipment such as heaters and air conditioners have often been located within the thermostat housing. These relays can be either non-latching or latching in well known examples in the prior art. The non-latching relays generate substantial heat as compared with latching relays in these applications. Non-latching relays are becoming more widely used for programmable thermostats. The more crowded thermostat housing will be unlikely to provide adequate ventilation to effectively remove this generated heat.
Once again consider the thermistor in the modern thermostat. Vents or openings are formed in the housing so that ambient air contacts the thermistor. The thermistor must be in continuous contact with a flow of ambient air so that the most accurate value for ambient air temperature is obtained. Although ambient air is allowed to flow over the thermistor, the temperature sensed by the thermistor for use by the thermostat control program and the temperature of ambient air are often significantly different. The thermistor can absorb heat by conduction or convection from electrical components within the thermostat housing. The sensed temperature of the thermistor in this case is higher than that of the ambient air. This causes the thermostat to operate environmental control equipment in error. Air conditioning equipment operates too much and heating equipment operates not enough.
The present inventor has found that the thermistor in modern thermostats absorbs heat by conduction from other electrical components through their common metallic electrical connections and through common mounting on a printed circuit board. The well known printed circuit board comprises a highly efficient heat transfer material in the glass fill used for its manufacture. While the glass fill provides exceptional support in a thin layer, electrical components mounted to such a board will absorb heat by conduction from remotely mounted heat sources. These heat sources can be other power dissipating components mounted on the same circuit board or can be other heat emitting surfaces exposed to the circuit board. Heat may be transferred to the circuit board by radiation or conduction.
Limited techniques are available for dissipating heat from components in high-density component operation. For example, fans may be employed to increase the flow of air across the surface of heated devices and components. Further, heat sinks may absorb dissipated heat and increase the surface area over which that heat may be dissipated.
However, these techniques are of little value in a modern day thermostat. Excessive cost and limited space for these components make them economically unavailable to reduce heat transfer to a thermistor locally mounted in a thermostat housing.
There is a need for a thermostat system where thermal isolation can be achieved for a thermistor locally mounted in a thermostat housing.